1. Field of the Invention
This invention relates to a composition of matter, a method for preparation of said composition, the use of the composition in a process of manufacture and the resultant article of manufacture. More specifically, this invention is directed to an improved solder paste formulation incorporating an organo-metallic formate complex, the method for the preparation of the solder paste incorporating such complexes, the use of the improved solder paste formulation in a soldering operation, and the printed circuit board manufactured with the improved solder paste of this invention.
2. Description of the Prior Art
The formulation of solder paste for use in electronics manufacture is a highly empirical art. A typical solder paste will typically contain at least three functional components: a solder alloy, a flux, and a vehicle (generally a polymer resin, within which the solder alloy and flux are contained). There are reportedly more than fifty (50) variables affecting solder paste performance; See Hwang, J. S. et al., "The Complex Concoction", Circuits Manufacturing, pages 73-78 (December 1987). Reportedly, this complexity is not merely the result of the number of variables or parameters, but, rather, the interaction of these variables or parameters with one another. Further complicating the formulation process are the variables which are introduced by paste handling and the re-flow process.
For example, the science of metallurgy will determine the selection of a solder alloy based upon proper solderability and joint integrity. Particle technology will dictate the physical characteristics of the alloy powder for desired dispensing and printing properties. The composition of the alloy powder also affects a paste's ability to solder within a given vehicle and flux system.
Chemistry fosters a better understanding of the chemical properties of the paste, such as the reactivity of the solder powder and the substrate, upon which it is applied, its reactivity at operating temperatures (this affects solderability), residue characteristics and joint integrity. This understanding must also extend to surface and interface phenomenon, as well as physical and thermal properties of individual chemicals and of the whole system. These factors have a significant effect on the paste solderability and residue properties. From a rheological standpoint, a material like solder paste is normally treated as a visco-elastic solid. Therefore, its flow properties under shear rates become necessary rheological information to predict its behavior in the environment of contemplated use. For example, a paste characterized by low thixotropy and yield point demonstrates good fine dot dispensability. In addition, elasticity and time effect on flow, as dynamically tested, are also useful to understanding rheology, hence the behavior of the paste. Flow behavior and elasticity can be used as fingerprints to control paste quality and batch to batch consistency.
In order for a solder paste to work effectively, that is, to form an electrically acceptable pattern on a circuit board, the surface interface between the solder and the substrate to which it is applied must be clean (i.e., free of residues, such as dirt, grease, and oxides). The flux components of the solder paste functions to remove/displace surface contaminates from both the solder alloy (solder balls) and the circuit board substrate, to insure that the solder wets the substrate surface at re-flow. The solder flux also functions to assist in heat transfer between the surfaces to be joined during the re-flow process. Generally, the flux performs its cleaning function by chemical/scavenging interaction of surface contaminants.
In practice, the flux, once having performed its function, must be removed; that is, the residues which remain from the flux must be removed in order to avoid corrosion of the surfaces upon which such residues remain. A number of fluxes require activators in order to operate properly at re-flow. One of the components (activators) of the flux, which can contribute to corrosion is halide (i.e. chloride salts). The relative acidity of different fluxes has been reported in the literature, see, for example, Jeanie S. Hwang, Solder Pastes in Electronics Packaging, Van Nostrand Reinhold, New York (1989).
The main contaminants to be removed by fluxes are the reaction products of the base metals (printed circuit board substrates) with air (ie. oxides, hydroxides, and sulfides). This is typically achieved during the re-flow process wherein the flux combines with the various contaminants and/or affects their displacement in order to ensure the good wettability of the solder and the substrate of the printed circuit board. Where the flux and/or contaminants are not volatilized completely, residues remain on the circuit board which can detract from the performance (electrical) of the circuit board or otherwise adversely affect the durability of the circuit board over time. In order to avoid these problems associated with the flux residues and other contaminants remaining after re-flow, the resultant circuit board is washed with organic solvents such as the chloroflorocarbons and othe similar environmentally offensive materials.
The "solder cream" formulation, described in UK Patent 2,198,676, is representative of the efforts to eliminate solder residues and thus obviate the need for a separate cleaning step employing the relatively offensive agents referred to hereinabove. This patent achieves its objectives through the incorporation into the solder cream of one or more of the following aliphatic carboxylic acids (propionic acid, oxalic acid, adipic acid, malic acid, maleic acid and citric acid and aromatic carboxylic acids, for example, salicylic acid). While formulations, such as that described in the above patent, have been proven to some extent to be efficacious, improvement still is required to the effect the requisite degree of cleanliness demanded in precise and exacting electronics applications. Thus, the need continues to exist for a solder paste which is both compatible with screen printing manufacture of printed circuit boards and yet provides the requisite properties to ensure that minimal residue will remain on the printed circuit board subsequent to re-flow and thereby preserve the electrical integrity of the circuit board and its long term durability.